Heretofore, magnetism has been used in an anti-theft system to prevent, for example, the stealing of books from a library or commodities from a department store (Refer to Japanese Patent Post-Examination Publication No. 58-53800 and U.S. Pat. No. 4,510,489). In such a system, a marker having a width of 1-2 mm and being formed of an amorphous alloy thin ribbon is previously attached to every book or every commodity. In order to lawfully remove such a commodity or book, for example, the commodity or book is delivered to a customer outside of a marker detector after a lawful procedure (i.e.. paying for the commodity or signing out the book) is completed in a reception or adjustment office. On the other hand, a commodity or the like which is illegally or unlawfully taken out is detected through the marker previously attached to the commodity or the like by detecting the magnetic field of a frequency having a harmonic relationship to the magnetic field of a specific frequency applied to a detection region set up at an entrance or exit. In short, the stealing of the commodity is checked.
FIG. 1 is a typical circuit diagram showing an example of the aforementioned magnetic anti-theft system. In the drawing, the reference numeral 1 designates an oscillator for generating an AC current of a frequency f. The reference numeral 2 designates a notch filter formed to remove a specific frequency from the alternating current and arranged to transmit the AC current to an oscillation coil 4 through an amplifier 3. The reference numeral 5 designates a receiving coil. The receiving coil 5 and the oscillation coil 4 form a detection region 6. A lock-in amplifier 7 and a signal processing circuit 8 are connected in series to the receiving coil 5.
According to the aforementioned construction, a specific harmonic component can be outputted through the lock-in amplifier 7 when, for example, the marker 9 is disposed within the detection region 6 to which an incident magnetic field Ha is applied, in the presence of a bias magnetic field Hb (the geomagnetisim). The specific harmonic componet thus outputted can be converted into a visible or audible signal through the signal processing circuit 8. Accordingly, a wrongful act can be easily exposed or prevented by connecting a patrol light or buzzer to the succeeding stage of the signal processing circuit 8.
As another method, there is known an anti-theft system using a marker formed of an amorphous alloy thin robbon having a relatively large electromechanical coupling coefficient. According to this system, the marker is excited with an AC current after being biased magnetically, so that the stealing of the commodity or the like can be detected through the presence of the marker by measuring frequencies of resonance and non-resonance.
Similar methods other than the aforementioned methods are known as anti-theft sensor system using a marker formed of an amorphous alloy thin ribbon. The most important feature in these systems is that the soft magnetic alloy, used as the marker, has excellent magnetic characteristics. In other words, the requirement for the magnetic characteristics of the marker used in the anti-theft sensor system is as follows: (1) the magnetic permeability is large; (2) the magnetizing curve is angular and (3) the coercive force is relatively small.
FIG. 2 shows the dependence or relationship of the output voltage on or with the incident magnetic field in the case where the marker, formed of a soft magnetic alloy, is present within the detection region 6 in the system of FIG. 1. In the FIG. 2, a designates a tertiary harmonic component (3f) and b designates a secondary harmonic component (2f). In the system, the value 2f-3f is detected so that the presence of the maker within the detection region can be identified. Accordingly, the detection sensitivity of the marker increases as the area surrounded by the curve a and the x-coordinate axis increases relative to the area surrounded by the curve b and the x-coordinate axis.
FIG. 3 shows an example of the anti-theft sensor marker. In FIG. 3, the reference numeral 10 designates a soft magnetic alloy ribbon the reference numeral 11 designates a first support member, for example, formed of paper, and the reference numeral 12 designates a second support member, for example, formed of polypropylene. The soft magnetic alloy ribbon 10 is fixed between the support members 11 and 12 through an adhesive agent. In general, an adhesive agent is also applied to the rear surface of the member 11 so that the marker can be easily fixed to a commodity or the like.
The requirement for the characteristics of the soft magnetic alloy used in the marker is as follows: (1) maximum magnetic permeability is large; (2) the angular rate of the magnetizing curve is large; (3) the coercive force is relatively small; and (4) magnetostriction is small.
Permalloy and amorphous alloy are known as soft magnetic alloys having the aforementioned characteristics (for example, as disclosed in Japanese Patent Post-Examination Publication No. 58-53800, Japanese Patent Unexamined Publication No. 58-39396, and the like). Almost all of the magnetic anti-theft sensor markers which have been put into practice employ one of the aforementioned soft magnetic alloys.
As described above, the prior art type anti-theft sensor markers have employed either permalloy or amorphous alloy. However, in the case of permalloy, the soft magnetic characteristics deteriorate remarkably due to bending stress, and therefore the range of use is limited because the marker within the detection region cannot always be detected. On the other hand, in the case of amorphous alloy, the deterioration of the soft magnetic characteristics due to bending stress is considerably less than that in the case of permalloy. Accordingly, the use of amorphous alloy is superior to the use of permalloy in this respect. However. the soft magnetic characteristics of amorphous alloy as a marker is unsatisfactory. More particularly, in order to reduce the deterioration of the soft magnetic characteristics due to bending stress, amorphous alloy, in general, mainly contains Co and has a relatively small saturation magnetostriction constant(.lambda..sub.s).
As a result, the costs associated with the Co amorphous alloy are expensive.